The resolution of our newly developed scanning transmission ion microscope (STIM) will be increased to several hundred A. The potential of the instrument to provide useful contrast related to chemical composition will be explored. A systematic program of measurements involving charge fractions, ratios of elastic-to-inelastic scattering cross sections and energy loss spectra will be carried out on specimens of known chemical composition. The measurements will be extended to various pure macromolecules of biological importance, that can be spread in thin layers or prepared in thin crystals. The performance of the STIM in providing secondary electron images (SIM mode) will be studied. The recently developed multichannel capability of our instrument will be tested by taking micrographs of biological specimens using various contrast mechanisms simultaneously. Examined will be preparations of herpes simplex virus, human metaphase chromosomes and erythrocyte membranes. From these observations, the potential of the STIM-SIM in biomedical research, at a resolution in the several A will be evaluated. BIBLIOGRAPHIC REFERENCES: W. H. Escovitz, T.R. Fox and R. Levi-Setti. Proton vs. Electron Scanning Transmission Microscopy: The Ratio of Elastic to Inelastic Cross Sections. EFI Report 76-30. Ultramicroscopy 1, (1976) p. 383-386. W.H.. Escovitz, T.R. Fox and R. Levi-Setti. Results with a Proton Scanning Microscope and Comparison of Proton Damage with Electron Damage. Invited paper presented by R. Levi-Setti at the Workshop on Short Wavelength Microscopy, The Annals of the New York Academy of Sciences. New York, February 23-2, 1977, in press.